A buried interconnection structure is formed by burying an interconnection material in an interconnection opening such as an interconnection trench or hole formed in an insulative film by an interconnection forming technique referred to as a damascene technique (single-damascene) technique and a dual damascene technique. However, in a case of using copper (Cu) as a main material for the interconnection, since copper tends to be diffused more in an insulative film compared with a metal such as aluminum (Al), the surface (bottom and lateral side) of the buried interconnection is covered with a thin barrier metal film such that the buried interconnection comprising copper is not in direct contact with the insulative film thereby suppressing or preventing copper in the buried interconnection from diffusing into the insulative film. Further, a barrier insulative film for an interconnection cap comprising, for example, a silicon nitride film is formed on the upper surface of an insulative film formed with the interconnection opening to cover the upper surface of the buried interconnection thereby suppressing or preventing the copper in the buried interconnection from diffusing from the upper surface of the buried interconnection into the insulative film.
In these recent years, the distance between the buried interconnections has been decreased along with increase in the integration degree of semiconductor devices. This increases a parasitic capacitance between the interconnections to cause signal delay and generate cross-talk between the adjacent interconnections. Accordingly it is desirable to decrease the parasitic capacitance between interconnections. Materials of low dielectric constant are used for the insulative film between interconnections. Further, JP-A No. 2001-85519 discloses a technique of forming an inter-layer insulative film such that the interconnection is formed in an inverted tapered shape and an air gap is formed in the space between the interconnections. The capacitance between the interconnections is intended to be decreased by the air gap. In addition, JP-A No. 2003-297918 discloses a technique of forming an air gap between interconnections.